The invention relates to a rotary filter for concentrating fibre suspensions, such as diluted pulp. The filter is of the known type which comprises a plurality of annular disks each composed of sector-shaped filter boxes. The disks are mounted in spaced relationship on a common horizontal shaft which is journalled in a vessel intended to contain liquid to be filtered, and the shaft is connected to driving means. Each of the filter boxes has at least two opposite walls made of wire gauze or filter cloth, and the inner end of each box has an outlet for filtrate communicating with a discharge conduit which extends longitudinally through the shaft. The discharge conduits are connected to suction means so that liquid is exhausted from the filter boxes immersed below the liquid level in the vessel. At the same time fibres will get deposited in layers on the outsides of the boxes. Above the liquid level the layers of fibres are removed from the filter surfaces, usually by means of water jets, and the thickened pulp thus released drops into collecting vessels from which it is transported to a vat.
In the known filters of this kind, narrow collecting troughs are stationarily mounted above the rotary shaft within the spaces between the filter disks. However, the dimensioning of such collecting troughs has always involved a problem, particularly due to the requirement that the filter apparatus should be useful for filtering suspensions of different concentrations. If the clearances between the brims of the troughs and the adjacent filter surfaces are made small to secure an effective picking up of the thickened pulp, there might result the risk of jamming of the fibre layers deposited on the filter surfaces, so that the rotation of the rotary shaft is stopped. This may occur particularly when the concentration of the suspension to be filtered is higher than normal. On the other hand, if the clearances are widened to avoid such risk, the picking up of the fibre concentrate will often be less effective.